Phosphorylation-dehosphorylation of enzymes controlling rate-limiting steps is one of the most important mechanisms by which cellular metabolism is controlled by hormones and other regulators. Glycogen synthase and phosphorylase kinase are two of such enzymes regulated by this mechanism in response to hormonal actions. These enzymes can be phosphorylated and dephosphorylated, respectively, by multiple forms of protein kinases and phosphatases. It is not entirely understood how hormones affect the activities of the various protein kinases and phosphatases. Previous studies have defined the action of glucagon and Beta-adrenergic agonists via the pathway involving cAMP and cAMP-dependent protein kinase. However, the actions of other hormone-mediated phosphorylation systems have yet to be correlated directly with the action of certain kinases. Tumor-promoting phorbol esters mimic the action of some hormones which regulate glycogen synthase activity in isolated hepatocytes. The pleiotropic responses elicited by these phorbol esters are presumably through binding and activation of their receptor, which has been tentatively identified as a phospholipid-dependent and calcium-activated protein kinase (protein kinase C). This protein kinase is ubiquitous in eukaryotes and seems to play a pivotal role in mediating the actions of signal-induced breakdown of inositol phospholipids. Protein kinase C from rat brain has been purified to near homogeneity in high yield. This enzyme phosphorylates glycogen synthase without causing its inactivation, in contrast to the effect of exposing intact hepatocytes to tumor-promoting phobol esters. These findings indicate that the actions of other mediators in addition to protein kinase C must be necessary to express the effect of phorbol esters. Polyclonal and monoclonal antibodies against protein kinase C have been prepared for immunocytochemical studies, and the regulation of protein kinase C activity by autophosphorylation is under investigation.